This invention relates to the manufacture of fibrous web articles such as paper. It is particularly directed to methods and apparatus for controlling feed of foamed fiber-containing slurry to a moving foraminous support on which the fibers are deposited to form a continuous web.
In a conventional papermaking operation, an aqueous slurry (furnish) of wood and/or other fibers is discharged through the outlet (slice opening) of a distributor (headbox) onto the surface of a continuously moving foraminous support (Fourdrinier wire), or between facing surfaces of two such moving supports, for deposit of the fibers thereon so as to constitute a continuous fibrous web, which is dried and commonly subjected to other subsequent treatments. Especially for the manufacture of products such as tissues, close control of the linear velocity of the slurry jet discharged through the slice opening is important, because the relationship between the jet velocity and the linear speed of the forming (Fourdrinier) wire determines the orientation of the fibers in the web, and this in turn governs such product properties as tensile ratio (longitudinal vs. transverse tensile strength) of the web.
Flow control is also important for operations in which a plurality of fiber-containing slurry flows are discharged through a multichannel or multislice headbox onto a moving foraminous support so as to produce a stratified web comprising (for instance) a bulk-providing central stratum sandwiched between thinner but tougher outer strata. Such operations are described, for example, in U.S. Pat. No. 4,086,130 (Justus). With a multichannel headbox, it is necessary to control the ratio of the flow rates in the respective headbox channels and the total flow rate through the headbox as well as to control the relationship between slurry jet velocity and speed of the forming wire.
When a simple liquid-fiber slurry is used, flow control in papermaking operations is reasonably straightforward. It is sometimes preferred, however, to employ as the slurry vehicle (in which the fibers are dispersed) a mixture of gas and liquid, such as a dispersion of air bubbles in water with a suitable surfactant, as described in U.S. Pat. No. 3,716,449 (Gatward et al.). For example, a foamed slurry is advantageous in some procedures for forming a bulky web of fibers that have been rendered anfractuous (kinked) by milling, because water tends to relax the desired kinked state of the fibers, and a foamed vehicle reduces the exposure of the fibers to water. Control of flow rate and headbox jet velocity of foamed slurries has heretofore presented substantial difficulties, since the variability of foamed (gas-liquid) slurry vehicles in respect of such fluid properties as density, viscosity, and compressibility prevents application thereto of conventional flow control techniques.